Raw sewage, as received by sewage treatment plants, invariably includes a certain quantity of hard inorganic particles comprising sand, silt and similar type of materials which are generally termed grit. The sewage treatment plant includes various apparatus components and processes that are intended to operate on and treat the organic components, but their functioning and particularly the operating life of various apparatus components are adversely affected by the presence of the grit which is entrained in the fluids comprising raw sewage. Specifically, apparatus in a sewage treatment plant includes fluid pumps that are used to induce flow of the material through the different processes and these pumps are adversely affected by the existence of the hard inorganic grit particles which cause excessive and rapid wearing of their moving components. Consequently, a first procedural step in most sewage treatments plants must, of necessity, include a grit tank having the primary objective of removing at least the larger size grit particles from the sewage. It is essentially impossible to remove all inorganic materials, but the finer grains that cannot be readily or effectively removed within accepted operating economics have a lesser tendency to affect the treatment process or the operation of the mechanical apparatus such as the pumps. In general, particles that are of a size which will pass through a 65 to 70 mesh screen and have a size not greater than 0.2 mm do not present such a serious problem as to pump and machinery wear that they cannot be accepted for processing with the sewage.
It is the function of a grit tank to initially receive the raw sewage and effect this seapration of the undesirably large size grit and enable its removal for separate disposal. This separation is accomplished by providing of a grit tank through which the sewage flows at a predetermined rate relative to the grit size and its quantity such that there will be an opportunity for grit of the unacceptably larger sizes to settle out and deposit on the bottom of the tank where it may be collected for removal. Grit tanks have a generally elongated shape with the sewage flowing in at one end and out at the opposite end. During the course of transport of the sewage through the grit tank, it is desired that a circulatory type of current in the form of a helical path be induced to increase the time period during which the sewage will be retained within the grit tank to better enable the grit to settle out onto the bottom of the tank. The settling of the grit is dependent upon time as it is a gravity type of action. Thus, the longer the period of time that sewage is retained in the grit tank, the greater the opportunity for the grit to settle out of the sewage flowing through the tank. Removal of grit by the simple expedient of effecting a separation by gravity is complicated by the practical fact that sewage is received on a continuing basis at different flow rates with the proportional concentration of sewage and grit also being a continually variable factor. During the course of any selected time period, such as a day, week or longer timeperiod, the raw sewage as it is received will have different proportions or percentages of grit to the other fluids and organic sewage components as well as have variations in the proportionate percentages of different grain sizes for the inorganic grit. Effective separation and collection of grit of greater than acceptable size thus depends in a substantial degree upon the velocity of the circulating currents generated within the grit tank as that is a primary factor determining whether the grit of the selected sizes will be retained for a sufficient time within the tank to permit its settling to the tank floor and subsequent collection. Consequently, it is important to be able to exercise control over the velocity of the circulating currents in order to accommodate the variation in grit size and the quantity of grit that is expected in the raw sewage received by the grit tank.
Attempts have been made to provide such control in order to better enable the separation of the grit. An example of a grit tank designed to achieve the general objectives, is illustrated in U.S. Pat. No. 3,215,276 granted Nov. 2, 1965 to A. C. Lind et al and assigned to Rex Chain Belt, Inc. The grit tank in that patented structure is of an elongated configuration having a generally rectangular cross-section and includes a relatively large chamber disposed in parallel relationship to a relatively small chamber that is located adjacent one longitudinal wall. A helical flow path circulation of the fluid is effected by introducing streams of air into the sewage along a longitudinal line adjacent a bottom wall and within the region of the relatively smaller chamber with that air then functioning as an airlift to cause upward flow of the fluid in the small chamber and thence transversely across the larger chamber. The result is produced, and the sewage must necessarily move through a relatively long path substantially greater than the actual physical longitudinal length of the tank and the sewage is consequentially retained in the tank for a greater length of time. Additionally, the structure disclosed in this patent includes an air distribution system wherein the amout of air that is introduced throughout the longitudinal extent of the tank is of varying quantities with the quantity decreasing successively from a relatively greater amount at the inlet end of the tank to a lesser amount at the outlet end. The effect of this variation in the quantity of air discharged throughout the length of the tank is that the circulatory velocity of the sewage is successively or sequentially decreased as the fluid progresses longitudinally through the tank so that the smaller particles will have a greater tendency to settle at the lower circulatory velocities as they approach the outlet end of the tank. The longitudinal wall or baffle that divides the tank into two chambers is mounted in a manner such that its lower edge is at a fixed elevation with respect to the bottom of the tank and the upper end or longitudinal edge will be a distance below the upper surface of the fluid within the tank.
In a grit tank of the type that is illustrated in U.S. Pat. No. 3,215,276, it is also essential that the circulatory current have a sufficient velocity as to move or displace grit that settles on the bottom of the tank to a collector trough or collection area from which it may be subsequently collected and transported to a discharge point. This circulatory current velocity must be of sufficient magnitude such that it will move grit particles that have settled onto the bottom floor of the tank and to also further aid in separation of any organic particles that may also settle out onto the tank floor along with the grit. The objective is to assure that the organic particles will be separated and again entrained in the fluid for continued transport through the tank and outwardly therefrom for further processing.